New Images Show Earliest Galaxies Forming at the Edge of the Universe
By studying images of the faintest galaxies ever detected, astronomers at the University of Durham have shown that galaxies formed when the universe was only 20-30 % of its present age - earlier than expected according to some popular cosmological theories. Members of the Durham Cosmology Group, led by Dr Tom Shanks, combined data from the Hubble Space Telescope - the 'Hubble Deep Field' - with new pictures from the 4.2-metre William Herschel Telescope (WHT) in the Canary Islands and from the 3.8-metre UK Infrared Telescope (UKIRT) on Hawaii. These new pictures have penetrated the depths of space further than any previous pictures taken with ground-based telescopes. The results are published in the scientific journal 'Nature' on 19 September 1996 (vol. 383, p. 236).
In the astronomical pictures that probe the greatest depths of the universe, a million faint galaxies can be seen in an area of sky the size of the full Moon. For some time, astronomers have been aware that many of these faint galaxies are blue in colour, and the fainter they are the more blue they look. The blue colour suggests that these galaxies contain many newly-formed stars. (Older collections of stars tend to be red.) They appear to be the first stars to be born when the galaxies came into existence.
It seemed likely that the faint blue galaxies are very remote. But the question so far unanswered has been just how far away and how old they are. Now the Durham team has used detailed information about the colour of the galaxies, obtained by using three different telescopes, to estimate their redshifts. The size of the redshift shows how fast the galaxies are receding and, through Hubble's Law, their distance and age.
It turns out that the faint blue galaxies are at a redshift (z) of 2, much higher than previously thought. This finding puts the epoch of galaxy formation early in the history of the universe, when it was only 20-30% of its present age. The light from these newly- formed galaxies has taken around 10 billion years to reach us.
The Deepest Ever Probes of Space
To obtain images of very faint galaxies, astronomers collect light from a single small patch of sky for many hours. Observations from different observing sessions can be added together in a computer. The WHT image for this research took 50 hours of observing time and the UKIRT infrared image took 30 hours. The Hubble Space Telescope (HST) 'Deep Field' was the result of 120 hours of observing a small speck of sky.
Being above the Earth's atmosphere, the HST can take sharper pictures than ground-based telescopes and the Hubble Deep Field goes about twice as deep into space as the WHT and UKIRT images. However, right up to the limit where the different pictures can be compared, counting how many galaxies there are in different brightness ranges gives the same consistent result.
Deep pictures such as these act as 'time machines', allowing astronomers to probe back in time as well as through vast distances in space. Travelling at 300,000 kilometres per second, light takes 8 minutes to reach us from the Sun and more than 4 years from the next nearest star. Light from remote galaxies takes billions of years to reach us. By revealing how galaxies looked in the past, these deep pictures allow cosmologists to test theories about how and when galaxies formed.
Estimating the Redshift of Faint Galaxies
Astronomers usually need to obtain spectra of galaxies to measure their redshifts. Such observations are difficult, if not impossible, for very faint galaxies. However, the combination of observations of numerous galaxies in different wavelength regions with three different telescopes is a good substitute. The HST observed its Deep Field in ultraviolet, blue and red light. The WHT image is in blue light and the UKIRT image in the infrared. With this technique, the Durham team can say that the typical redshift of one of the faint blue galaxies is about z=2.
How Did Galaxies Form?
The problem of how galaxies formed still remains. The new observations throw some further light on this question but do not settle it. To some extent the answer depends on whether the universe is open and will expand forever, or closed and headed for a recollapse to a Big Crunch. If it is open, the universe has a much bigger volume than if it is closed. If it is open, the high numbers of galaxies detected in the deep pictures may be consistent with the number density of galaxies in space at redshift, z=2, being roughly the same as it is at the present day. In this situation, a single faint blue galaxy at high redshift might evolve into a single galaxy much like the Milky Way by the present day.
In the scenario in which the universe is closed, because of its smaller volume, the numbers of galaxies detected in the deep pictures are too large to have the same number density of galaxies in space at z=2 as at present. If the universe is closed, there have to be more galaxies per unit volume of space at z=2 than around us now. In this case, either many of the z=2 galaxies have to merge together under gravity ultimately to become galaxies like the Milky Way or, alternatively, many of the galaxies have to fade and disappear by the present day to reduce galaxy numbers.
Currently the pros and cons of these various theories for how galaxies form and evolve are finely balanced. Either could be right on the basis of what is known so far. This question forms the subject of continuing research.
New observational technologies will help answer these and other questions in the future. They include Durham University's MARTINI instrument, which is already providing Space Telescope quality pictures from ground-based telescopes, and the two giant GEMINI 8-m telescopes, which the UK, with mainly US and Canadian collaboration, is currently building for completion in 1999.
The Durham Cosmology Group
The members of the Durham research team are Nigel Metcalfe (Senior Research Assistant/Computer Officer), Tom Shanks (Reader), Ana Campos (EC Research Fellow), Dick Fong (Senior Lecturer), and Jon Gardner (Senior Research Assistant). The Durham Cosmology group has been active in Deep Imaging/Galaxy Counts research since 1976. The group is very experienced in the techniques of measuring the light from faint galaxies and has led the way in faint galaxy research, first using machine-measured photographs to take detect galaxies with blue magnitudes (B) in the range 20-24 over the period 1976-1984. They then used electronic CCD detectors on 4-metre ground-based telescopes in the period between 1985 and 1996. Most recently, they used Hubble Space Telescope data to probe from B=25 to B=29. The group is also highly experienced in the techniques of analyzing the counts and colours of the faint galaxies detected in these deep images.
Hard copies of the deep pictures (in colour) from the William Herschel Telescope, the UK Infrared Telescope and the Hubble Space Telescope can be obtained from Tom Shanks (see below for contact information).
Computer files, in PostScript form, containing the deep pictures can be obtained by logging in to the Durham Starlink anonymous ftp account : star-ftp.dur.ac.uk and then, on UNIX systems, by giving the command cd/pub/nm/press. The command mget *.ps will then allow you to copy the PostScript files, together with a copy of this press release.
The deep pictures will also be shown on the world-wide web site: http://star-www.dur.ac.uk/~hjmcc/herschel
Figure 1. The Herschel Deep Field. This is the deepest picture of the Universe ever obtained from a ground-based telescope in the blue band and it was taken on the 4.2-metre UK/Dutch/Spanish William Herschel Telescope on the Canary Islands in a long, 50 hour exposure. The picture is 7x7 square arcminutes in area. The faintest galaxies visible on this picture are fainter than blue magnitude (B) 28. The colours in the picture represent the true colours of the galaxies and it can easily be seen that the faintest are predominantly blue, indicating that they are young galaxies seen in the process of formation when the Universe was only a fifth of its current age.
Figure 2. UK Infrared Telescope Deep Field. This is one of the deepest infrared pictures of the Universe ever taken and it was obtained on the 3.8-m UK Infrared Telescope in a long, 30-hour exposure. The picture is 1.2x1.2 square arcminutes in area an is a sub-area of the Herschel Field shown in Fig. 1. The faintest galaxies visible on this picture have infrared K magnitude 23. The colours in the picture represent the true colours of the galaxies and it can be seen that the faintest are predominantly blue, indicating the presence of young stars and therefore young galaxies.
Figure 3. The Hubble Deep Field. This is the deepest picture of the Universe ever obtained in visible light from either a space- or ground-based telescope and it was taken on the 2.5-metre Hubble Space Telescope with exposures of 25 hours in each of the ultraviolet, red and near IR bands and an exposure of 47 hours in the ultraviolet band. The faintest galaxies visible on this picture reach magnitude B=29. The colours in the picture represent the true colours of the galaxies. Because of the high resolution of the HST pictures, the measurements of galaxy colours are very precise and this allows new estimates of the redshifts of the faint blue galaxies to be made. The conclusion is that the faint, young, blue galaxies lie at z=2. This implies that the epoch of galaxy formation corresponds to z=2, when the universe was 20-30% its present age.
Contact for Further Information
Dr Tom Shanks, Department of Physics, University of Durham, South Road, Durham DH1 3LE, England.
Tel. 0191 374 2171; Fax. 0191 374 3749;
An electronic version of the paper and its Figures in PostScript form can be found by logging in to our Durham Starlink ftp account: star-ftp.dur.ac.uk as anonymous and then, on UNIX systems, by giving the command cd /pub/nm/nature. The command mget *.ps will allow you to copy the postscript files.
A copy of this press release and the deep pictures in postscript format can also be obtained by the same procedure, only changing the above command to cd /pub/nm/press.
This press release and the deep pictures and instructions as to how to obtain the paper will also be shown on the world-wide web site: